The invention is related to producing cement clinker.
In known processes for producing cement clinker, raw material fed into a rotary kiln is preheated and partially decarbonated in a multistage cyclone suspension preheater system and a precalciner by using the heat of combustion gases exhausted from the rotary kiln and precalciner. As the combustion gases and raw material mix, lime (Cao) in the raw material and sulfur dioxide (SO.sub.2) in the combustion gases react to form calcium sulfite (CaSO.sub.3). The calcium sulfite is formed in the preheater and in the main electrostatic precipitator of the stack. The calcium sulfite, in turn, reacts with oxygen in the preheater system to form calcium sulfate (CaSO.sub.4), if there is sufficient oxygen. If there is not enough oxygen in the atmosphere at the kiln's inlet, the calcium sulfate may decompose into lime and sulfur dioxide and leave depositions at the kiln's inlet. If there is an insufficient excess of oxygen in the rotary kiln, the calcium sulfate may decompose at temperatures of 1200.degree. Celsius. Similarly, if there is not enough oxygen in the preheaters, the calcium sulfite may decompose into lime and sulfur dioxide. This decomposition also leads to an increase in sulfur dioxide concentration in the gas in the kiln, which leads to depositions of calcium salts on the shells and walls of the preheater's cyclones and ducts. The level of deposit formation may be increased when the combustion fuel is a solid fuel high in sulfur (i.e., above 2%), such as petcoke, oil shale, and agricultural or industrial wastes, or a fuel oil high in sulfur content because of the resulting increased sulfur dioxide concentration in the kiln gas. The increased sulfur circulating in the gases causes an increase in the quantity of calcium sulfite. This may result in deposits to a level sufficient to close the kiln inlet, preheater, preheater cyclones, and ducts connecting the cyclones, thereby stopping production. The problem can be alleviated by extracting a fraction of the gas between the rotary kiln and preheater and sending it to a bypass tower. In the bypass tower, the gas is quenched with cooler atmospheric air and a dust rich in sulfur dioxide precipitates out. The desulfurized gas is then directed into the preheater, the result being an overall reduction in the concentration of sulfur dioxide in the gas in the preheater. This solution poses two significant problems: a loss in thermal energy and an environmental issue in disposing of the precipitated dust.
Alternatively, the oxygen can be controlled to ensure an excess oxygen concentration in the kiln and eliminate the need for a bypass tower. However, this potential solution is prone to problems associated with oxygen sensor reliability in a kiln environment, which is further reduced at the kiln inlet where oxygen concentration is even more important. At the inlet, the gas intake for oxygen analyzers can be filled by the dust circulating in the kiln environment. Because current oxygen sensors in the kiln environment may be unreliable, it is not practical to provide continuous control of cement clinker production using an oxygen sensor. To provide excess oxygen by merely increasing the flow of air through the kiln, precalciner, and preheaters may create other problems associated with reduced thermal efficiency and pressure loss.